Topoisomerase and Microtubule Inhibitors Flashcards
1
Q
What are topoisomerase inhibitors?
A
- DNA cutters.
- Interfere with DNA repair.
2
Q
What are the types of topoisomerase inhibitors?
A
- Camptothecin and derivatives.
- Anthracyclines.
- Podophyllotoxin derivatives.
3
Q
What are DNA topoisomerases?
A
- Important in DNA replication and RNA transcription
- Type I topoisomerases change the degree of supercoiling of DNA by causing ss breaks and re-ligation
- Subtypes IA-IIIalpha (top3A) 1A-IIIβ (top3B), 1B (top1)
- Type II topoisomerases cause ds breaks.
- Subtypes IIalpha (top2A), IIβ (top2B)
- Topoisomerases are generally present at elevated levels in tumors
- Resolution of the molecular structures of topoisomerases has opened new
avenues for drug development in this area.
4
Q
What are the targets of the currently marked cancer chemotherapeutics?
A
- The targets of the currently marketed cancer chemotherapeutic agents are topoisomerase I, IIα, and IIβ.
5
Q
What are the types of topoisomerase inhibitors?
A
- “Topoisomerase poisons” -inhibit the re-ligation step and locks enzyme into a “cleavage complex” - enhance the rate of cleavage.
- Competitive inhibition of the ATP binding site – only Type II topoisomerases – prevents ATP- hydrolysis drive enzymatic action (novobiocin; coumermycin).
- Inhibitors of DNA-topoisomerase binding (aclaruibicin; suramin).
- Inhibit ATP hydrolysis and DNA release at the last step (dexrazoxane) – Type II.
6
Q
What are the topo IB inhibitors?
A
- Camptothecin.
7
Q
What is camptothecin?
A
- Forms a stable ternary complex with the topo I-DNA complex
- Intercalation between the -1 and +1 DNA base pairs in the protein-DNA cleavage complex. Additional hydrophobic and electrostatic interactions stabilize the binding of the poison and prevent DNA re-ligation by the topoisomerase
- Causes DNA strand breaks which results in apoptosis
- Remarkable activity in preclinical trials (has high affinity, but in vivo has ow solubility and adverse drug reactions (therefore not ideal).
- Isolated from the bark and stem of Camptotheca
acuminata (Camptotheca, Happy tree), a tree native to
China used it as a cancer treatment in Traditional Chinese Medicine.
8
Q
How was camptothecin modified?
A
- Camptothecin was modified by biochemists to enhance pharmacokinetics and decrease adverse effects.
- Substitution of the A ring increases solubility while retaining cytoxicity.
- Irinotecan and topotecan: semisynthetic analogues of camptothecin.
9
Q
What is irinotecan used for?
A
- Prodrug.
- Colon cancer – FOLFIRI (5- FluOrouracil, Leucovorin, and IRInotecan).
10
Q
What is topotecan used for?
A
- Ovarian and lung cancer.
11
Q
Describe irinotecan metabolism.
A
- It can be metabolized to several substrates but metabolism by carboxylesterase (CE) forms the active metabolite SN-38 which is similar to camptothecin, but at this point its already in the body and distributed well (combat the low solubility issue).
12
Q
Describe irinotecan activation.
A
- Irinotecan is converted to an active metabolite (SN-38) by carboxylesterase.
- SN-38 is 1000 times more active than irinotecan itself.
13
Q
What happens when there is down regulation of carboxylesterase?
A
- Down-regulation of carboxylesterase → resistance.
14
Q
How is SN38 inactivated?
A
- SN38 is metabolized to an inactive form via
glucoronidation by UGT1A1. - There is a reduced function variant of UGT1A1 in ~10% of Caucasians → poor metabolism of Irinotecan → Irinotecan toxicity, as it cannot be excreted from the body in its SN-38 form.
- Irinotecan (CPT-11) is also inactivated by P450s (Cyp3A4).
- Increased expression of either Cyp3A4 or UGT1A1 can lead to resistance to this drug.
15
Q
What are anthracyclins?
A
- Antitumor antibiotics.
- Get in the way of the function of the topoisomerase enzyme.
- DNA-intercalating topoII inhibitors.
16
Q
What are the anthracyclins?
A
- Doxorubicin (adriamycin).
- Daunorubicin – Solid tumors, Leukemia, Lymphoma.
- Idarubicin – AML (Acute Myeloid Leukemia).
- Epirubicin – Breast cancer.
- Valrubicin – Bladder cancer.
17
Q
Describe the mechanism of action of anthracyclines.
A
- Inhibition of topoisomerase II (subtype-independent)
causing DNA strand breakage. - Intercalative binding to DNA.
- Partial unwinding of DNA.
- Much of the DNA is organized and folded into chromatin and maybe protected from this type of damage
18
Q
What are the side effects of anthracyclines?
A
- Free radical formation
- Generation of OH. and O2: DNA strand breaks and cell membrane damage.
- Contributes to their therapeutic effect, but these radicals are not exclusive to the tumor.
19
Q
Describe the toxicity of anthracyclins.
A
- ∗Myelosuppression.
- ∗Mucositis (GI irritation).
- Cardiac toxicity (acute arrhythmias and delayed cardiomyopathies) - due to free radical formation, cumulative and irreversible damage.
- Delayed cardiotoxicity is related to a patient’s cumulative lifetime dose.
- Mechanism not entirely known.
- Superoxide radicals, hydrogen peroxide.
- Disruption of calcium and iron levels in cells.
20
Q
How is anthracycline cardiotoxicity prevented?
A
- Dexrazoxane (also a topo II inhibitor) – FDA approved – indicated to reduce adverse cardiac effects in women on doxorubicin who have already received a total dose of over 300 mg/m2 – may be due to its iron chelating activity (antioxidant).
- Co-admin of carvedilol (adrenoceptor blocker) – also
antioxidant in cardiac tissue. - Use of anthraquinones (mitoxantron, pixantrone) – act like anthracycline drugs but with fewer adverse effects/toxicities.
21
Q
How does resistance anthracyclines develop?
A
- Increase in efflux pumps.
- Increase in inactivation pathways (GSTs).
- Increase in DNA repair.
- Increase in antiapoptotic proteins and decrease in apoptotic proteins.
22
Q
What is the most important resistance mechanism to anthracycline drugs?
A
- Increase in DNA repair.
23
Q
What are podophyllotoxin derivatives?
A
- Semi-synthetic glycoside derivatives of podophyllotoxin.
1. Etoposide.
2. Teniposide.
24
Q
Which podophyllotoxin derivative has better cell accumulation?
A
- Teniposide has better cell accumulation.
- Doesn’t get pumped out as easily by efflux pump (not a good substrate).
25
Q
What is etoposide?
A
- Non-intercalating inhibitor of topo II.
- Formation of a stabilized ternary complex - Etoposide-topo II-DNA.
- Single and double strand breaks in DNA.
25
Q
What are the clinical uses of etoposide?
A
- Testicular cancer.
- Lung cancer.
- Acute myeloid leukemia.
25
Q
What is the dose limiting toxicity of etoposide?
A
- Myelosuppression.
26
Q
How does etoposide resistance develop?
A
- Enhanced efflux.
- Decreased binding to topoisomerase II.
- Increased glutathione conjugation.
27
Q
What are the antimicrotubular agents?
A
- Vinca alkaloids.
- Taxanes.
28
Q
What are the vinca alkaloids?
A
- Vinblastine.
- Vincristine.
29
Q
What are the taxanes?
A
- Paclitaxel.
- Docetaxel.
30
Q
What are microtubules?
A
- Play a role in cellular structural support – Component of the cytoskeleton
- Dynamic structures
- Important roles in: Formation and function of mitotic spindles (cell division) and axonal transport of organelles.
31
Q
What do anti-microtubule agents do?
A
- Anti-microtubule agents interfere with formation and function of microtubules.
- Stabilizers and destabilizers.
32
Q
What is the difference between vincristine (VCR) and vinblastine (VLB)?
A
- VCR contains a CHO R-group while VLB contains a CH3 R group.
33
Q
Describe how VCR and VLB work.
A
- Bind to tubulin at the end of microtubules and to
tubulin dimers. - Blocks assembly of microtubules (Destabilizer).
- Causes dissolution of mitotic spindles.
- Inhibit organelle transport.
- Especially mitochondria in neural cells.
34
Q
What is VCR used for?
A
- Vincristine
– Acute lymphoblastic leukemia (ALL)
– Hodgkin’s lymphoma
– Pediatric solid tumors - Side effect: ∗Peripheral neuropathy (disfunction of peripheral neurons).
35
Q
What is VLB sed for?
A
- Hodgkin’s lymphoma
- Non-small cell lung cancer
- Side effect: ∗ Myelosuppression.
36
Q
How does resistance to VCR and VLB develop?
A
- Increase in efflux.
37
Q
What is paclitaxel (Taxol)?
A
- From the Western yew, Taxus brevifolia.
- Binds to tubulin enhancing its polymerization (Stabilizer) - prevents dissolution of microtubule.
- Discrete bundles of stable microtubules.
- Inhibition of cell replication.
38
Q
What is paclitaxel used for?
A
- Ovarian and breast cancer.
- Lung cancer.
39
Q
What is docetaxel?
A
- Higher affinity for tubulin binding site than paclitaxel.
- Side effects: ∗MYELOSUPPRESSION (neutropenia).
- Hypersensitivity reactions.
40
Q
What is docetaxel used for?
A
- Ovarian and breast cancer.
41
Q
What is noscapine?
A
- Isoquinoline alkaloid found in opium latex.
- Sigma opioid receptor agonist.
- Used as a non-sedating antitussive in various countries - available OTC in Canada.
- Can be used orally.
- Under investigation for use in the treatment of several cancers, particularly for prostate cancer (at doses 100-fold higher than those effective as an antitussive).
- Fewer side effects than other microtubule disruptors (nausea and abdominal discomfort).
- Not as good a substrate for efflux transporters.
